Abstract. The processes of aerosol sulfate formation are vital components in the
scientific understanding of perturbations of earth's radiative balance via
aerosol direct and indirect effects. In this work, an analysis of the
influence of changes in oxidant levels and sulfur dioxide oxidation pathways
was performed to study the underlying pathways for sulfate formation.
Sensitivities of this constituent were calculated from a series of
photochemical model simulations with varying rates of NOx and VOC
emissions to produce variations in oxidant abundances using a photochemical
model (CMAQ) that covers the eastern US for part of the ICARTT 2004
campaign. Three different chemical mechanisms (CBIV, CB05, and SAPRC99) were
used to test model responses to changes in NOx and VOC concentrations.
Comparison of modeled results and measurements demonstrates that the
simulations with all three chemical mechanisms capture the levels of sulfate
reasonably well. However, the three mechanisms are shown to have
significantly different responses in sulfate formation when the emissions of
NOx and/or VOC are altered, reflecting different photochemical regimes
under which the formation of sulfate occurs. Also, an analysis of the
oxidation pathways that contribute to sulfur dioxide conversion to sulfate
reveals substantial differences in the importance of the various pathways
among the three chemical mechanisms. These findings suggest that estimations
of the influence that future changes in primary emissions or other changes
which perturb SO2 oxidants have on sulfate abundances, and on its
direct and indirect radiative forcing effects, may be dependent on the
chemical mechanism employed in the model analysis.